1. Field of the Invention
The present invention relates to an image pickup apparatus, an image pickup method, and a program therefor for generating a combined image signal with a wide dynamic range by combining a long-exposure image signal and a short-exposure image signal.
2. Description of the Related Art
In image pickup apparatuses using a solid-state image pickup device such as a charge coupled device (CCD), the quantity of light to be input into the image pickup device (light quantity of exposure) is adjusted by controlling an aperture and an electronic shutter speed. That is, the light quantity of exposure is adjusted when images are captured. More specifically, a bright scene is captured with a reduced light quantity of exposure so as to avoid overexposure caused by saturation of a signal output from the image pickup device. Conversely, a dark scene is captured with an increased light quantity of exposure so as to avoid underexposure of such an output signal.
However, when a scene with a strong contrast is desired to be captured (for example, image capturing of a backlit subject or simultaneous image capturing of an indoor scene and an outdoor scene) using a solid-state image pickup device with an insufficient dynamic range, even if the light quantity of exposure is adjusted, overexposure may occur in a bright portion of the scene due to the saturation of the bright portion and underexposure may occur in a dark portion of the scene. As a result, both of these portions are not appropriately reproduced.
As a method of overcoming such a difficulty, for example, Japanese Unexamined Patent Application Publication No. 6-141229 and its corresponding U.S. Pat. No. 5,455,621 disclose a method of separately obtaining bright area information and dark area information by using two different electronic shutter speeds in a field or changing the electronic shutter speed on a field-by-field basis, and combining the obtained items of information into a single image.
This method is employed by apparatuses capable of capturing an image with a wide dynamic range (wide dynamic range cameras). Japanese Unexamined Patent Application Publication Nos. 2002-84449 and 2004-120205 (corresponding US Published Application 2006/0033823 A1) disclose apparatuses and methods for improving the quality of the above-described combined image.
Wide dynamic range cameras generally have two types of image capturing modes, a combined image capturing mode in which wide dynamic range image capturing is performed and a normal image capturing mode in which no wide dynamic range image capturing is performed. In many cases, monitoring cameras are constantly operated in the combined image capturing mode. This is because of the following reasons. A monitoring camera plays the role of monitoring a wide area by automatically or manually rotating (panning and tilting) the camera itself. Because there are places where a wide dynamic range is indispensable and places where such a wide dynamic range is not indispensable, and because this indispensability may change with time, it is difficult to predict which image capturing mode is more appropriate. The image capturing mode can be switched automatically or manually in accordance with the state of a subject. However, this results in a problem that the quality of a moving image is degraded because an output video image signal suddenly changes when a signal processing algorithm is switched in accordance with the image capturing mode.
In the case of a monitoring camera that monitors a wide area, it is assumed that the luminance of a subject greatly varies from one monitoring area to another. It is thus more preferable to continue the combined image capturing mode than causing changes in a video image due to frequent changes in the image capturing mode for each monitoring area.
Further, the monitoring camera is requested to capture both a scene with a strong contrast (e.g., image capturing of a backlit subject or simultaneous image capturing of an indoor scene and an outdoor scene) where combined image capturing is necessary and a scene with a weak contrast where the combined image capturing is not necessary (a scene suitably captured in the normal image capturing mode) in the combined image capturing mode. The monitoring camera is requested to capture high-quality images of both scenes.